Various ceramic uses require low-melting ceramic frit which advantageously can be applied to a substrate and fused into a ceramic coating at a temperature of about 550.degree. C. to 700.degree. C. Low-melting and durable glass frit formulations are highly desirable for uses such as vitrifiable glass decorating colors, glass-to-metal sealing, and vitreous coatings. Low-melting frits have an incipient fusion point less than about 475.degree. C. as measured by the dilatometer method of coefficient of thermal expansion. The durability of a glass frit can be measured by exposing glass frit powder to various attacking agents for a specified test period and at a specified temperature. For instance, a glass frit powder sample of minus 100 plus 200 mesh particle size, exposed to distilled water at 100.degree. C. for one hour and exhibiting total solubility of frit in water of less than 0.6% by weight, would be considered durable.
It is well known in the art of ceramics that the two properties of low-melting and high durability are generally opposing physical properties, and hence, frit materials simultaneously exhibiting both low-melting and high durability are difficult to achieve. Conventional approaches to compounding frits which are simultaneously low-melting and durable include lead borosilicate system such as disclosed in U.S. Pat. Nos. 3,258,350; 2,642,633; and 3,404,027; or lead zinc borosilicate systems such as disclosed in U.S. Pat. Nos. 3,873,330 and 3,258,350. Lead borosilicate systems, however, are based on highly toxic lead oxide as a major constituent which must be carefully handled to avoid ingestion or inhalation. Accordingly, a need exists for nontoxic, lead-free frit systems which are both low-melting and highly durable. Some alkali borosilicate glasses are adequately low-melting, but are not durable, in addition to exhibiting very high expansion. Other alkali borosilicates, as used in Pyrex and Kimax brand glasses, are durable glasses, but they exhibit high-melting properties. Blending or an admixture of such borosilicates does not provide the desired low-melting and durability characteristics. Alkali phosphate glasses or arsenic-selenium-tellurium-antimony glasses provide adequate low-melting properties but exhibit very poor durability to the extent that such frits are soluble in water at moderate temperatures. Zinc borosilicates such as disclosed in U.S. Pat. No. 3,113,878 can provide moderately durable and moderate melting characteristics; however, such systems do not produce satisfactory low-melting, durable frits particularly useful in applications requiring low-melting and high durability properties simultaneously.
It now has been found that a certain range of compositions within the frit system comprising alkali, zinc oxide, boron oxide, phosphorus oxide, silicon oxide, titanium oxide, and fluorine surprisingly provides highly desirable lead-free ceramic frits exhibiting low-water solubility, high durability and low-melting characteristics. These and other advantages of this invention will become more apparent from the Detailed Description of the Invention.